Restrict proposer registration and challenge submission for beta launch

[ChaitanyaKonda]

Nightfall is designed to have multiple block proposers and challengers.
The beta launch will be done with Polygon as the honest and only block proposer and challenger while awaiting audit of the PNF for the safety of user's funds.

Block proposer is restricted to ensure only good blocks are proposed.
Challenger is restricted to ensure good blocks are not successfully challenged.

Definition of Done:

• Only Polygon as the block proposer and challenger
• No changes to the code inside registerProposer or challenge functions
• Contract changes should be upgradeable to accept registration of any proposer and challenge from any challenger